1. Field of the Invention
This invention relates to a washing machine provided with a control device executing torque control for an electric motor developing torque used to carry out wash, rinse and dehydration operations.
2. Description of Related Art
Automatic washing machines have conventionally been provided which comprise a brushless DC motor driving an agitator (or pulsator) and a rotating tub in a wash step and only the rotating tub in a rinse step and a dehydration step. An inverter circuit is provided for driving the brushless DC motor in many types of the above-mentioned washing machines. Voltage applied to the motor is increased or decreased so that torque developed by the motor is controlled according to a driving condition of the motor.
FIG. 22 shows an example of control system for a three-phase drive motor of the aforementioned conventional automatic washing machine. The control system is composed of a microcomputer, for example and includes functional blocks of PI (proportional-integral) control 1, wash pattern output section 2, UVW converter 3, initial pattern output section 4, pulse width modulation (PWM) signal generator 5, position detector 6 and the like. The PWM signal generator 5 delivers PWM signals of respective phases to an inverter circuit 8 driving an electric motor 7. A Hall sensor 9 is incorporated in the motor 7 for detecting a position of a rotor. The Hall sensor 9 carries out position detection for two (U and V) of three phases, delivering position signals to the position detector 6.
The PI control 1 performs PI control for a rotational speed of the motor 7 on the basis of a target speed command ωref in a dehydrating operation and a detected speed ω of the motor 7. A control for controlling an operation of the washing machine delivers the target speed command ωref to the PI control 1, whereas the control delivers the detected speed ω to the PI control 1. The PI control 1 delivers a duty command and a phase command for a PWM signal to the UVW converter 3. The wash pattern output section 2 delivers a duty command and a phase command in a wash operation to the UVW converter 3, instead of the PI control 1. The UVW converter 3 converts the commands delivered from the PI control 1 or the wash pattern output section 2, into voltage commands of the respective phases U, V and W, delivering the voltage commands to the PWM signal generator 5. The initial pattern output section 4 delivers a 120-degree energization pattern signal to the inverter circuit 8, instead of the UVW converter 3, when the motor 7 starts from a stopped state.
The above-described control system has the following problems. A rotational speed of the motor 7 is proportional to torque developed. However, the developed torque is not proportional to the voltage when the control is performed by increasing or decreasing the applied voltage as described above. As a result, a difference is likely to occur between the target speed command ωref and the detected speed ω, whereupon the control becomes unstable. Furthermore, since a motor speed variation is increased in the wash operation (0.2 seconds from 0 to 150 rpm, for example), the PI control cannot be applied to the wash operation and accordingly, the PI control 1 needs to be switched to the wash pattern output section 2.
Furthermore, the inverter circuit 8 includes upper and lower arm side switching elements such as insulated gate bipolar transistors (IGBTs). A short-circuit current flows when both arm side switching elements are simultaneously turned on. A simultaneous off time or a dead time is provided in order that the short-circuit current may be prevented. In the dead time, the switching elements of both arms are simultaneously turned off when the elements are switched between the on state and off state. As the result of provision of the dead time, the current supplied from the inverter circuit 8 to each phase winding of the motor 7 undergoes waveform modulation.
A minimum time needs to be ensured as the dead time. Accordingly, an adverse effect on the output current waveform becomes larger as a carrier wave frequency for the pulse width modulation is increased. For example, 6 μs is required for on and off times in order that a dead time of 3 μs may be ensured. A ratio of the dead time to a carrier wave period is 3% when the carrier wave frequency for the pulse width modulation is at 5 kHz (period of 200 μs). The ratio is 10% when the carrier wave frequency is at 16 kHz (period of 62.5 μs). The carrier wave frequency is generally set at or above 10 kHz in the washing machines so that an audible noise produced by a pulse width modulated wave is reduced. Consequently, an increase in the adverse effect of the dead time upon the output current waveform cannot be avoided. More specifically, the modulation due to the dead time distorts the output voltage of the inverter circuit 8 and accordingly the output current waveform. The distortion of the output current waveform results in variations in the developed torque. Consequently, a cogging torque is developed with rotation of the motor, resulting in noise and vibration or oscillation.